Hilda and the Mountain King (Hildafolk #6) by Luke Pearson
From Nobrow Press:
See what perils await our beloved blue-haired adventurer in the sixth book of Luke Pearson’s widely praised series.
We rejoin our heroine for her latest adventure just as she awakes to find herself… in the body of a troll! Her mum is worried sick, and is perplexed by the strange creature that seems to have taken Hilda’s place. Now, both of them are in a race to be reunited before Ahlberg and his safety patrol get the chance to use their new secret weapon to lay waste to the trolls, and Hilda along with them!